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Pt100

ivan
Prodigy 40 points
Other Parts Discussed in Thread: ADS1148, ADS1248


Hello,

What chip (configuration) would you recommend for measuring 2 Pt100 probes (wire length less than 10 cm =  2-wire connection?).
The probes would be A grade  {±(0.15 + 0.0020 | t |)}  (or even BAND 3 (1/3 DIN) ±(0.10 + 0.0017 | t |)) for measuring in range from 0°C to 50°C.
Is it possible to measure the 2 probes and also their resistance difference?
Also i don't know much about RTD brands ( manufacturers ) who have Pt100 probes, if you could recommend me some that would help a lot.
For now i'm interested in www.farnell.com/.../1643984.pdf .
I'm planing to make something like this:
Is my current choice suitable.
Best regards
  • 0
    TI__Expert 7220 points
    Hi ivan,

    Our group does not have any parts that interfaces with RTD's. I searched the E2E forums and it seems like there are some parts in the precision ADC group that can interface with an RTD. I will move this topic to their forum.

    -Michael Wong
  • 0
    TI__Guru* 93655 points
    Ivan,


    Depending on the accuracy that you are looking for, I would recommend something like the ADS1248 (or ADS1148 if you are looking for a 16-bit device). If you look in the current version of the ADS1248 datasheet, we've recently re-written it to include some example applications, including one for a 3-wire RTD.

    Read through the example. Additionally, there is the following reference design from which this example was based:

    www.ti.com/.../slau520a.pdf

    If you read it carefully, there should be enough information to get you started. It's important to realize the source of errors when working on a design such as this, so you want to be able to follow through the design and the equations shown in the writeup for the reference design.

    Also, there is an ADS1248EVM that exists and you should be able to use that to take some measurements with it to get a proof of concept. You can find it here:

    www.ti.com/.../ads1248evm-pdk



    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 40 points
    Thank you for your answer it really helped me,

    If i wanted to measure 2 Pt100 would i connect them like this (basically the reference design for measuring 0-100 C):
    http://i.imgur.com/7OmWVb3.png (sorry for the bad shematic)

    If i wanted to measure the difference in resistance of the 2 Pt100 how could i connect them? And lastly would i need to change the design completely if i want to use 2 wire Pt100 .
  • 0 in reply to ivan
    TI__Guru* 93655 points
    Ivan,


    It looks like the schematic that you've provided would work to measure two 3-wire RTDs. You use AIN0-AIN3 to drive the RTD (and lead resistances) and measure it back comparing it with REF0. Then you use AIN4-AIN7 to drive a second RTD and measure it back comparing it to REF1.

    It's a standard setup. However, I would remove R6. It looks like you're using it for hardware compensation. This may work, but in practice it's difficult. You would need a precision, low drift resistor. Any errors in resistance and drift contributes to an error in the measurement. I would also lower the series resistances for the inputs and the reference (R5, R7, R8, R10, for example) to something below 5k. I there is some amount of input current to any of these inputs and I think you should keep any series resistance to less than 10k. The values you have should work, but I like to keep them lower.

    Using this to measure a 2-wire RTD, you wouldn't need to drive the second IDAC input (AIN3 for the first RTD), but you would need to connect the X1-2 point to the X1-3. You will have some series lead resistance as an error, and I don't think this would work with hardware compensation. Regardless, it shouldn't be hard to measure a two-wire RTD with a slightly modified setup.


    Joseph Wu